The disclosure of Japanese Patent Application No. 2000-082762 filed on Mar. 23, 2000 including the specification, drawings and abstract is incorporated herein by reference in its entirety.
1. Field of the Invention
The invention relates in general to an apparatus for controlling a transmission of a vehicle such as, for example, an automotive vehicle, and more particularly to an apparatus for controlling a transmission having a plurality of gear ratios provided in a vehicle that also includes an internal combustion engine and an electric generator which is disposed downstream of the transmission and which is capable of effecting regenerative braking so as to convert the energy of the braking into electric energy (that can be stored in a battery). The apparatus may assure smooth running of the internal combustion engine as well as efficient conversion of the regenerative braking energy into the electric energy.
2. Description of Related Art
A motor vehicle of the type powered by an internal combustion engine (gasoline engine or diesel engine) is usually equipped with a battery for power supply to energize various electronic components. This battery is charged by an electric generator (alternator) provided on the vehicle. In recent years, development and marketing of a hybrid vehicle (HV) equipped with a hybrid power train system have been in progress, in an effort to protect the environment and to improve the fuel economy of the vehicle. The hybrid power train system employs a combination of two different types of drive power sources such as a combination of an internal combustion engine and an electric motor (commonly referred to as xe2x80x9cmotor/generatorxe2x80x9d and abbreviated as xe2x80x9cMGxe2x80x9d). In the hybrid power train system, the internal combustion engine and the electric motor are selectively used depending upon a specific running condition of the vehicle. The hybrid power train system permits highly smooth and highly responsive control of the vehicle drive power, while making a good use of different characteristics of the internal combustion engine and the electric motor, with their disadvantageous aspects being compensated for by each other. Such a hybrid vehicle is also equipped with a battery for operating the MG as an electric motor. This battery is charged with an electric energy generated by the MG operated as an electric generator. More specifically, the electric generator is driven by a drive force produced by the internal combustion engine, or by a kinetic energy of the vehicle under deceleration, which kinetic energy is utilized by the electric generator for regenerative braking.
The regenerative braking with the electric generator (MG) takes place during deceleration of the vehicle, so as to convert the kinetic energy of the vehicle into an electric energy to be stored in the battery. The energy conversion efficiency is maximum if the entire amount of the kinetic energy of the vehicle can be converted into the electric energy. In this respect, the drive shaft connected to the drive wheels of the vehicle and the MG (electric generator) are connected to the engine through at least a transmission. Accordingly, the kinetic energy of the vehicle during deceleration is partly consumed by an action of the engine so-called xe2x80x9cmotoringxe2x80x9d, which creates a drag on the power train. The motoring of the engine results in an energy loss due to friction (engine friction being proportional to the square of the engine speed), so that the amount of the kinetic energy available for consumption by the electric generator is made smaller than the entire amount of the kinetic energy, and the amount of electric power that can be generated by the electric generator is accordingly reduced.
In order to increase the amount of electric power that can be generated during regenerative braking with the electric generator, there is known a method in which a clutch disposed between the drive shaft and the engine is released upon regenerative braking, so as to prevent the motoring of the engine, thereby avoiding an otherwise possible loss of the kinetic energy. With this method, the amount of the kinetic energy of the drive shaft that can be used by the electric generator may be increased to improve the regenerative braking efficiency. There is known another method in which the transmission having a plurality of gear positions (speed positions) that provide respective different gear ratios or speed ratios is automatically shifted up to the highest-gear position or highest-speed position (e.g., fifth-gear or sixth-gear position) upon regenerative braking, to lower the revolution speed of the engine connected to the transmission through the drive shaft, for reducing the engine friction and the resulting energy loss, to improve the regenerative braking efficiency. It is noted that the highest-gear position is defined as the position having the highest ratio of the output speed of the transmission to the input speed of the transmission. JP-A-8-251708 discloses a technique relating to the latter method. According to this technique, the shifting action of the transmission is controlled to select the appropriate gear ratio that enables the electric generator to operate in an optimum condition with the highest efficiency.
However, the method of releasing the clutch to disconnect the engine from the electric generator suffers from a speed reduction of the engine down to its idling speed or stalling of the engine. Therefore, this method requires a rise of the engine speed and an engaging action of the clutch, upon subsequent transition of the vehicle running state from the decelerating state into an accelerating state. These requirements cause a delay of the vehicle accelerating action with respect to an operation of the accelerator pedal (i.e., a poor response of the power train to an increase in the operating amount of the accelerator pedal), and prevent smooth acceleration of the vehicle, unexpectedly to the vehicle driver. On the other hand, the method of shifting the transmission up to the selected high-gear position or high-speed position may cause the engine speed (rotating speed of the drive shaft multiplied by the gear ratio of the transmission) to be lowered below a certain lower limit while the vehicle running speed is lowered during the deceleration. In this event, it is difficult to re-start (re-fire) the engine because its speed is lower than the lower limit. Further, the deceleration (or stopping) of the vehicle with the transmission placed in its highest-gear position when the vehicle is required to be shifted down toward the lowest-gear position requires the transmission to be accelerated again (to be re-started or launched). Thus, this method also suffers from a delay of the vehicle acceleration with respect to the accelerator pedal operation (i.e., a poor response of the power train to the accelerator pedal operation), preventing smooth acceleration of the vehicle, unexpectedly to the vehicle driver.
It is therefore an object of the invention to provide an apparatus for controlling a transmission provided in an automotive vehicle that includes an electric generator which is disposed downstream of the transmission and which is capable of effecting regenerative braking during a decelerating phase of the vehicle, which apparatus is arranged to assure smooth transition of the vehicle running state from the decelerating phase to an accelerating phase by re-starting of an internal combustion engine, as well as efficient conversion of the regenerative braking energy into electric energy.
The above and/or other objects may be achieved according to one aspect of the invention, which provides a transmission control apparatus of a vehicle that includes an internal combustion engine, a transmission connected to the internal combustion engine and having a plurality of gear positions with respective gear ratios, and a generator disposed between the transmission and drive wheels, which generator is capable of generating electric power through regenerative braking during deceleration of the vehicle. The apparatus includes a detector that detects a revolution speed of the internal combustion engine, and a shift controller that places the transmission in a highest gear position selected from one or more gear positions that enable the engine revolution speed to be maintained at a level that is not lower than a predetermined lower limit above which the engine can operate by itself (i.e., be re-started), when the generator generates electric power through regenerative braking.
According to the apparatus of this aspect of the invention, the transmission is automatically shifted, upon deceleration of the vehicle, to a highest gear position selected from one or more gear positions which enables the internal combustion engine to maintain its speed at a level not lower than a predetermined lower limit above which the engine can operate by itself. This arrangement not only enables the internal combustion engine to run at a speed not lower than the predetermined lower limit, but also permits the revolution speed of the internal combustion engine to be kept close to the predetermined lower limit, due to the automatic up-shift action of the transmission. Thus, the instant arrangement makes it possible to reduce the friction of the internal combustion engine and minimize the energy loss during the regenerative braking, resulting in an increase in the regenerative braking efficiency. Further, since the revolution speed of the internal combustion engine is kept high enough to enable the engine to be re-started, the transition of the running state of the vehicle from the decelerating phase to the accelerating phase (starting phase) can be smoothly effected with a high degree of response, as desired by the vehicle driver, without the conventionally required operations to control the power train, such as an engaging action of the clutch, an increase of the engine speed, and a down-shift action of the transmission.
In one preferred aspect of the invention, the shift controller determines whether the revolution speed of the engine is higher than the predetermined lower limit, and shifts down the transmission by one gear position when the revolution speed of the engine is not higher than the predetermined lower limit.
In another preferred aspect of the invention, the vehicle further includes a clutch disposed between the transmission and the engine, and the shift controller operates to release the clutch so as to disconnect the internal combustion engine and the transmission from each other, when the selected highest gear position is the lowest one of the plurality of gear positions of the transmission.
In the above aspect of the invention, the disconnection of the internal combustion engine from the transmission eliminates the friction of the internal combustion engine, and reduces the energy loss, permitting efficient regenerative braking with a kinetic energy transferred from the vehicle drive wheels. In addition, the transmission is shifted down to its lowest gear position before the internal combustion engine is connected again to the transmission, so that the vehicle can be smoothly accelerated (or started), without the conventionally effected down-shift action of the transmission from the highest-gear position.